Apparatus and method for manufacturing an integrated circuit

ABSTRACT

The present invention relates to an apparatus for manufacturing an integrated circuit ( 10 ) having a thick film metal layer ( 14 ). A layer of metal paste ( 14 ) is applied via an application means ( 24 ) onto a heat-conducting substrate ( 12 ). The metal paste ( 14 ) includes metal particles of a predetermined size. An RF generator ( 16 ) selectively inductively couples RF energy ( 18 ) into the metal paste ( 14 ). The predetermined size of the metal particles of the metal paste ( 14 ) corresponds to a coupling frequency of the RF energy ( 18 ), for heating the metal particles. In this way the metal particles of the metal paste ( 14 ) are heated with only a small fraction of the power of conventional processes, and without the need to pre-sinter the metal paste ( 14 ).

This application claims the benefit or priority of and describesrelationships between the following applications: wherein thisapplication is a continuation of U.S. patent application Ser. No.14/222888 filed Mar. 24, 2014, which is a continuation of U.S. patentapplication Ser. No. 13/258,105, filed Sep. 21, 2011, which is theNational Stage of International Application No. PCT/IB2010/051295, filedMar. 24, 2010, which claims the priority of foreign applicationEP09156417.9 filed Mar. 27, 2009, all of which are incorporated hereinin whole by reference.

The present invention relates to an apparatus and method formanufacturing an integrated circuit having a thick film metal layer.

Present integrated circuits, such as solar cells for a solar panel, arecurrently manufactured by a process wherein a metal paste is applied toa substrate, and the entire assembly is heated in order to fuse, melt orsinter the metal particles in the paste and thereby create the desiredcircuitry. A sufficient amount of energy is applied to heat both themetal paste and the substrate up to the melting/sintering temperature ofthe metal. Beneficial interaction between the substrate and conductingmaterial occurs at these high temperatures, but prolonged interactiondecreases performance due to substrate damage and/or substrate propertychanges.

The limits in heating and cooling rate are normally governed by the heatpower transfer to the total substrate with the metal paste.Conventionally, the total heat capacity of a substrate and metal needsto be considered to calculate a temperature increase for a given powerinput per second, and the heat capacity of, for example, a siliconsubstrate, is much larger than the heat capacity of the metal paste,e.g. a silver paste, arranged on the substrate. This heat capacity ratioensures that much more power is needed, using the conventional methods,to get the same temperature increase over the same time period as if themetal were heated alone.

DE 100 41 889 A discloses a procedure for thermally changing theelectrical properties of a semi-conducting coating material.

DE 102006005026A discloses an electrically conductive coating ofsintered particles on a glass substrate. The sintered particlesdisclosed are nano-particles of ITO. The glass and ITO are heatedcapacitively in a resonant cavity using microwaves between 300 MHz and30 GHz.

US Patent Publication 2005/0087226 discloses an electrode-arrangingmethod for thin films on non-flat substrates. The method uses inductiveheating in the range of several kHz to 1 MHz, which requires highlyconductive, pre-sintered materials. The substrate and electrode materialare both heated to the eutectic temperature of the substrate andelectrode material.

An object of the invention is to provide an apparatus and method forrapid heating and cooling of a metal paste on a heat-conductingsubstrate for an electronic device having improved electricalperformance and significant energy savings.

Another object of the invention is to provide a passive method of rapidcooling of a metal paste on a substrate, wherein the cooling rate isfaster than with the active cooling methods used in the case ofconventional infrared heating.

A further object of the invention is to provide a method for selectivelycoupling radio frequency (RF) energy with metal particles in a metalpaste on a substrate, wherein only the heat capacity of the metal andthe small energy losses to the surrounding substrate need to beconsidered, wherein significantly less power must be delivered to heatthe metal very quickly to a desired temperature as compared toconventional methods.

A subsequent object of the invention is to provide a method formanufacturing an integrated circuit having a metal layer wherein thesubstrate itself is not heated to the melting/sintering temperature ofthe metal, wherein only the metal layer needs to be cooled.

In a first aspect of the present invention, an apparatus formanufacturing an integrated circuit having a thick film metal layer isproposed, including an application means for applying a layer of metalpaste on a heat-conducting substrate, the metal paste having metalparticles of a predetermined size, and an RF generator for selectivelyinductively coupling RF energy into the metal paste, wherein the metalparticles of the predetermined size are selectively inductivelycouplable with the RF energy, the predetermined size of the metalparticles corresponding to a coupling frequency of the RF energy, forheating the metal particles.

In a further aspect, a method for manufacturing an integrated circuithaving a (sintered/molten) thick-film metal layer is proposed, includingapplying a layer of metal paste on a heat-conducting substrate, whereinthe metal paste includes metal particles of a predetermined size, andselectively inductively coupling RF energy into the metal paste from anRF generator, wherein the predetermined size of the metal particlescorresponds to a coupling frequency of the RF energy, and the metalparticles of the predetermined size are selectively inductivelycouplable with the RF energy, for heating the metal particles.

The present invention provides an apparatus and a method formanufacturing an integrated circuit having a (sintered/molten)thick-film metal paste on a substrate. The method is accomplished in afast and energy-efficient manner, due to selective inductive coupling ofthe metal particles in a metal paste on the substrate so that most ofthe inductive energy goes into heating the metal particles directly. Thesubstrate may receive some energy from the metal particles, but is notcoupled with the RF energy, or is coupled in a very limited manner,which results in a fast heating process which requires little energy,especially as compared to conventional processes.

The conventional processes require that the electrode material is firstsintered before any heating step, whether inductive or capacitive, inorder to create a highly conductive layer. At the low frequenciesdisclosed in the conventional art for inductive heating, the effect ofthe induction heating is very limited, and not very fast, in particulardue to the requirement for heating both the electrode material and thesubstrate together. Further, in the case of conventional processes usingcapacitive heating with microwaves, a resonance cavity is required, dueto the nature of capacitive heating and microwaves.

According to the present invention, pre-sintering of the electrodematerial is unnecessary because of the higher frequency which couplesinto the individual silver particles. The highly-conductive layer is notrequired at all. The efficiency of coupling is also greater. Because ofthis greater efficiency, the speed of heating is significantly higherthan conventional technologies, including inductive heating done atlower frequencies.

A wide range of frequencies is known for use in capacitive heatingprocesses, which are used to heat non-conducting materials as well asconducting materials. Inductive heating, for conducting materials, is acommon technology at frequencies of several 10's or 100's of kHz.However, induction at a range of 2-200 MHz, or, in particular, about 27MHz is not known because of a number of complications (such as electricbreakdown due to the high voltage required, and controlling the heatingprocess) that need to be dealt with, as described below.

At low frequencies, e.g. 10-1000 kHz, it is common to use a coil that isas large as the product to be heated. This is for a non-localizedheating scenario. Occasionally, a smaller coil is chosen to increase thefield strength that can be applied for more localized heating. Ofcourse, in order to be effective, the conventional arts teach that theapplication of this low-frequency energy requires that the patterns tobe heated, e.g. electrode material, is highly conductive. This requiresthat the electrode material is pre-sintered prior to inductive heating.

In the intermediate frequency range, e.g. 2-200 MHz, in particular 27MHz, between the conventional inductive frequencies and microwaves, botha local and a non-local heating approach can be used. However, the useof a localized field, and a small coil, has strong advantages: bettertemperature field homogeneity can be achieved more easily, and the fieldstrength can be much higher. Further, the coupling of the RF field tothe individual metal particles in the metal paste means thatpre-sintering is unnecessary. The present disclosure uses the RF fieldto sinter the electrode materials as they are selectively heated.

Low-frequency inductive heating, as provided in the conventional art,sets strict demands on the geometry of the electrode-material patternson the substrate in order to control the temperature field and providehomogeneity. The presently disclosed heating arrangement is much lesssensitive to the geometry of the patterns to be heated.

In contrast to the conventional art, the present disclosure uses alocalized field from an RF generator that may be generated by a smallcoil. This arrangement has two important advantages:

Increased homogeneity of the heating process; and

Very high field strengths are possible without the use of extremely highvoltages.

According to an embodiment, the substrate, due to its greater mass, hasa higher heat capacity than the metal in the metal paste. This makes itpossible for the substrate to act as a heat sink to provide rapidcooling of the metal layer.

According to a second embodiment, the substrate may include silicon,gallium-arsenic compounds, germanium, indium-tellurium compounds, andcopper-indium-gallium-sulfur compounds. These materials provide theadvantage of a large inductive heating efficiency difference (and heatcapacity) difference between themselves and many of the metals used inintegrated circuit manufacturing.

According to another embodiment, the substrate may act as a heat sinkfor rapidly cooling the selectively-coupled metal. The advantage ofselectively coupling the metal on the substrate means that when a metalis selectively coupled, the substrate will remain relatively coolthroughout the coupling period. Thus, a large capacity to absorb thetemperature and energy differences between the substrate and the metalis available, wherein the metal may be cooled very rapidly.

According to a subsequent embodiment, the metal paste may comprise metalparticles of silver, aluminum, copper, stainless steel and otherconducting metals appropriate for use in integrated circuits. A widevariety of metals may be used to advantage depending on the intendedapplication of the integrated circuit and the heat capacity of thesubstrate.

According to another embodiment, the metal particles in the metal pasteare selectively coupled at a very high frequency in the range of 2 to200 MHz. Selective coupling of the metals means that the substrate isnot affected thermally in a direct manner and remains relatively coolwith respect to the metal particles. The selected frequency range ishigher and more effective in coupling than typical frequencies used forinduction, and are significantly below the microwave frequencies, whichrequire resonance cavities and extensive shielding. This frequency rangeprovides sufficient energy to selectively couple metal particles in themetal paste, without over-penetrating the substrate materials orrequiring extensive shielding arrangements, such as with microwaves.Some simple shielding may be required only to prevent disturbances inelectronic appliances.

According to another embodiment, the metal particles of the metal pasteare micro-particles, which are couplable with RF energy of about 27 MHz.This provides the advantage that a predetermined, uniform size of metalparticles may be used with a narrowly-selected frequency band to providecontrollable coupling without numerous undesirable considerations, e.g.over-penetration of the RF energy or excessive heating of the substrate.A properly sized particle complements nicely with a particular frequencyband to provide a controlled method for manufacture. The micro-particlesare larger than nano-sized particles, which would require microwave RFenergy for coupling. Microwave RF energy would require a shieldingarrangement to protect the surroundings.

According to a further embodiment, a substrate table may be provided tomove the substrate having the metal paste under the RF generator at apredetermined rate, wherein the RF energy is distributed in apredetermined manner. This provides the advantage of uniform andcontrolled distribution of the RF energy, for selective heating ornon-heating of the metal paste on the substrate. The RF energy may bemodulated relative to the position of the substrate to enhancetemperature homogeneity. It was observed that the heating under constantRF power and constant substrate speed was not homogeneous enough in thesubstrate movement direction, in some situations. The effect of heatconduction into the substrate caused a gradient in temperature increasewhen RF power was constant over the whole substrate travel length. Thisis caused by cooling in the first part, as a large, relatively coolsubstrate is available to act as heat sink, while in the last part, onlya small piece of the substrate is relatively cool and the heat sinkcapacity of this piece is smaller. In one embodiment, modification ofthe RF power delivery is done by electrical or mechanical adjustment ofthe RF power output, controlled by signals indicating the substrateposition. By adjusting the RF power to the coil while the substratepasses beneath it, the power loading can be “programmed” for eachsubstrate position individually, enabling increased homogeneity orintended non-homogeneity.

According to a subsequent embodiment, the apparatus may include asubstrate heater for pre-heating the substrate, wherein the conductionproperties of the substrate are changeable with a temperature change.This provides the advantage of changing or manipulating the physicalproperties of the substrate, e.g. conduction properties, with thetemperature increase. An advantage with this step is that a moreconductive substrate, e.g. heated silicon, even when slightly heated,provides a more efficient heat sink arrangement for cooling the metalpaste. In addition, the pre-heating step increases process stability.Silicon is non-conductive at low temperatures, and therefore does notheat up in the applied inductive field. At higher temperatures, however,it becomes conductive and starts to couple RF energy. When the siliconsubstrate is not pre-heated, the sudden change in the absorbingproperties of the silicon can render the process uncontrollable, causingvery inhomogeneous heating. In contrast, if the substrate is pre-heatedto >400° C., the absorbing properties change very gradually, and can beaccounted for in the process settings. The amount of energy absorbed perunit material is much less for silicon than for silver, even at thistemperature, so the selective-heating principle is still fullyapplicable. This effect may also be significant if the substrate is adifferent semi-conductive material, for instance GaAs, Ge, InTe, CuInGaSor any other material with a band gap property.

According to a subsequent embodiment, the layer of metal paste isarranged with a predetermined three-dimensional geometry, wherein thetemperature of the inductively-coupled metal particles is manipulable bythe geometry. When the RF energy is carefully modulated, wherein thesize of the metal particles in the metal paste is complementary with theRF energy frequency, a specific controllable penetration depth may beaccomplished for the RF energy, and different temperatures may beachieved for different portions of the metal paste layer, depending onthe predetermined three-dimensional geometry of the metal paste.

Also integrated circuits with multiple (2 or more) layers of thick filmmetal layers, with non-conductive layers in between, can be heated usingthe invention with RF heating. Connections with the 2 layers using viasare also possible.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent from andelucidated with reference to the embodiment(s) described hereinafter. Inthe following drawings

FIG. 1 illustrates a top view of a substrate for an integrated circuithaving a pattern of a conductive metal paste thereon, in accordance withan embodiment of the invention.

FIG. 2 illustrates a side sectional view of a substrate for anintegrated circuit having an applied pattern of a metal conductive pasteunder an RF coil, in accordance with an embodiment of the invention.

FIG. 3 illustrates a low-frequency induction-heating arrangement, inaccordance with conventional methods.

FIG. 4 illustrates a medium-frequency induction-heating arrangement, inaccordance with an embodiment of the invention.

FIG. 5 illustrates a temperature-change curve for a substrate, inaccordance with an embodiment of the invention.

FIG. 6 illustrates a perspective view of an apparatus for manufacturingan integrated circuit having an applied metal conductive paste, inaccordance with an embodiment of the invention.

FIGS. 7A-7D illustrate top and side views of elements for a multi-layerintegrated circuit having an applied metal paste, in accordance with anembodiment of the invention.

FIG. 8 illustrates the coupling frequencies and correspondingpenetration depth for different metals, in accordance with an embodimentof the invention.

FIG. 1 illustrates an arrangement of an integrated circuit 10 having asubstrate 12 and an applied metal paste 14 arranged in a particularpattern. The thick film metal layer 14 is applied to the heat-conductingsubstrate 12 via an applicator 24, as shown in FIG. 6. The metal paste14 includes metal particles of a predetermined size. As shown in FIG. 2,an RF generator 16, such as a coil, is used to selectively inductivelycouple RF energy (18) into the metal particles of the metal paste 14, inorder to heat the metal particles.

The present invention provides an apparatus and a method formanufacturing an integrated circuit 10 having a thick-film metal paste14 on a substrate 12. The method is accomplished in a fast andenergy-efficient manner, due to the selective coupling of the metalparticles in a metal paste 14 on the substrate 12 so that most of theinductive energy 18 goes into heating the metal particles directly. Thesubstrate 12 may receive some thermal energy via heat conduction fromthe metal particles, but is not coupled, or is coupled only to a verylimited degree in case of a pre-heated silicon substrate, with the RFenergy, which results in a fast heating process which requires verylittle energy, especially as compared to conventional processes.

The conventional processes require that the electrode material is firstsintered before any heating step, whether inductive or capacitive, inorder to create a highly conductive layer. At the low frequenciesdisclosed in the conventional art for inductive heating, the effect ofthe induction heating is very limited, and not very fast, perhaps due tothe requirement for heating both the electrode material and thesubstrate together. Further, in the case of conventional processes usingcapacitive heating with microwaves, a resonance cavity is required, dueto the nature of capacitive heating and microwaves. FIG. 3 illustratesthe application of a low-frequency field, e.g. 100 kHz, to a layer of asintered electrode material S having a thickness d. Due to the sinteringprocess, the sintered electrode material S will exhibit an inductioncurrent I. The induction current I through the electrode material Scauses heating of the electrode material S. Heating is the by-product ofpre-sintering the electrode material S, the application of thelow-frequency field and the induction of a current I in the electrodematerial S. This process takes too much effort and the same results canbe achieved more easily via a different process.

According to the present invention, and as illustrated in FIG. 4,pre-sintering of the electrode material is unnecessary because of thehigher frequency, e.g. 2-200 MHz, field which couples into theindividual metal, e.g. silver, particles 24 having a particle size Q.The particle size Q is typically much smaller than the thickness d ofthe electrode material of FIG. 3. As a result of the coupling ofindividual particles Q, the highly-conductive electrode layer S is notrequired at all. The efficiency of higher-frequency coupling of FIG. 4is also greater than conventional processes. Because of this greaterefficiency, the speed of heating is significantly higher thanconventional technologies, including inductive heating done at lowerfrequencies.

The substrate 12 may have a higher heat capacity than the metal paste14. Accordingly, the substrate 12 may be a heat sink to provide forrapidly cooling the selectively inductively-coupled metal paste 14.Rapid cooling complements the selective coupling, and selective heating,wherein relatively little energy is used to heat the substrate and asignificant amount of energy is saved as compared to the conventionalart.

The substrate 12 may be made from a number of materials includingsilicon, gallium-arsenic compounds, germanium, indium-telluriumcompounds, copper-indium-gallium-sulfur compounds and other compounds ormaterials having heat capacities and conductive properties similar tothe foregoing materials. A larger number of materials may be used withthe disclosed method to manufacture integrated circuits and thick-filmmetal layers in a very energy-efficient manner.

The metal paste 14 may include a variety of metals, including silver,aluminum, copper and stainless steel, or other metals capable of beingprocessed according to the disclosed method. A wide variety of metalshaving different properties may be used according to the disclosedmethod for producing integrated circuits or thick-film layers in anenergy-efficient manner.

The metal paste 14 may be selectively coupled with the RF energy 18 at avery high frequency. The complementary RF frequency and metal particlesize provide more controllability. According to another embodiment, thefrequency of the RF energy 18 is around 27 megahertz. This particularfrequency range provides the advantage of sufficient penetration of themetal paste 14 via coupling while avoiding the necessity for RFshielding, as would be the case for microwave energies.

The metal particles of the metal paste 14 are sized so as to beresponsive to the RF energy 18. Appropriate selection of the RF energy18 with a complimentary particle size results in efficient, selectiveheating of the metal particles in the metal paste 14, without excessiveheating, and associated energy waste, in the substrate 12. In a furtherembodiment, the metal particles are micro-particles, e.g. 5-50 μm, whichare responsive to RF energy 18 in the range of 2-200 MHz, in particularabout 27 MHz. However, the particles may be even larger than 50 μm,depending on the particular metal selected. The micro-particles may beabout 12 μm diameter. The micro-particles are far larger than thenano-particles that would require the use of microwave energy-typefrequencies and the corresponding shielding requirements. Thus, thecombination of the micro-particles with a 27 megahertz frequency is bothefficient and easily controlled.

As illustrated in FIG. 6, the substrate 12 having the metal paste 14 maybe moved beneath the RF coil 16 at a predetermined rate. Thisarrangement provides for even distribution of the RF energy 18 from theRF coil 16 into the metal paste 14.

The substrate 12 may be preheated to effect a change in the conductionproperties of the substrate 12. Certain materials exhibit significantchanges in their conductive properties with changes in theirtemperature, e.g. silicon, as illustrated in FIG. 5. FIG. 5 illustratesthat the temperature change AT of the substrate 12 due to passingthrough the RF field depends on the initial temperature To of thesubstrate 12. For the example of a silicon substrate, the characteristictemperature K is about 400° C. That is, the temperature at which theconductive properties have changed to the point where they can be usedeffectively to advantage is about 400° C. Below about 400° C., thecharacteristic, e.g. conductive, properties are not very great. Withrespect to silicon, at about 400° C., the RF energy coupled into thesilicon equals the heat loss via conduction/convection. Thus, above thischaracteristic or “critical” temperature, the RF field can induce asignificant temperature increase, while below this temperature itcannot.

Thus by raising the temperature of the substrate 12 to a point above thecharacteristic temperature K, the increase of—and stabilityof—conduction of the substrate 12 results in a more efficient heat sinkarrangement of the substrate 12 with respect to the selectively-coupled,e.g. selectively-heated, metal paste 14. That is, when the substratematerials are appropriately selected and matched to the metal pastelayer 14, the heated substrate 12 is better and more constant atabsorbing the thermal energy of the metal paste 14 than when thesubstrate 12 is cool.

FIG. 6 illustrates an apparatus for manufacturing an integrated circuitwith a thick-film metal layer having a substrate table 20 to support thesubstrate 12. The substrate table 20 may be used to move the substrate12 under the RF generator 16 at a predetermined rate, for theapplication of RF energy in a predetermined manner. In addition, theapparatus may include a substrate heater 22 for pre-heating thesubstrate 12 in a manner to take advantage of the change in conductiveproperties of the substrate 12 above its characteristic temperature K.By modulating the RF power to the RF generator 16 as the substrate 12passes beneath it, the power loading P, or programmable power wave form,can be determined and applied for each position of the substrate 12.This may be used to achieve improved homogeneity of non-homogeneity ofRF application, and resultant temperature, as desired.

The layer of metal paste 14 may be arranged on the substrate 12 with apredetermined three-dimensional geometry, as shown in FIG. 2, whereinthe temperature of the coupled metal paste 14 may be manipulated as aresult of the geometry. The RF energy necessary for coupling with themetal paste 14 is calculable so that it is not excessive with respect toover-penetration of the metal paste 14, and will penetrate the metalpaste 14 to a desired depth. Various heating and cooling arrangements ofthe metal paste 14 in particular areas of the substrate 12 may becreated through various geometries, e.g. thicknesses, widths andlengths, of the applied metal paste 14 on the substrate 12 to achievethe desired conductive results.

In one example, the invention relies on selective coupling of the RFenergy 18 into a metal paste 14 containing silver. In this case, onlythe heat capacity of the silver and the small energy loss to thesurrounding substrate 12 needs to be considered for delivering RFenergy, which results in a requirement for significantly less power toheat the silver very quickly to a desired temperature, via coupling,than would be required in conventional processes. However, once heated,the silver would need to be cooled quickly as well to preventdetrimental effects to the combined electrical properties of substrateand silver conductor. This problem is solved due to the selectivecoupling of the metal paste 14 via RF energy 18, in that the substrate12 is not heated to high temperatures, so only the silver needs to becooled.

The limits in heating and cooling rate are normally governed by the heatpower transfer to the total substrate, including the metal paste 14.Conventionally, the total heat capacity of the substrate 12 and metalpaste 14, e.g. silver, needs to be considered to calculate a requiredtemperature increase for a given power input per second. The heatcapacity of the substrate 12 in this example, e.g. silicon, is muchlarger than silver's heat capacity, which is present atop the substrate12. This disparate ratio ensures that much more power is needed to heatthe metal paste when conventional methods are used to achieve the sametemperature increase in the same amount of time.

Conversely, roughly the same calculation can be applied in determiningthe necessary cooling power using conventional methods. Much more poweris required to cool the substrate 12, e.g. silicon, when usingconventional methods than if the metal paste 14, e.g. silver, isselectively coupled using RF power 18. As described above, the benefitin this case is that the substrate 12 itself acts as a heat sink to coolthe metal paste 14 many degrees for every few degrees temperatureincrease in the substrate 12. This means that, effectively, the metalpaste 14 on the substrate 12 can be cooled virtually instantly to thesubstrate's temperature.

These small dimensions of the silver particles Q, e.g. micro-particles,in the metal paste 14 may be selectively coupled only with very high RFfrequencies in the range of 1-50 MHz, or in particular, about 27 MHz.The penetration depth of the RF energy 18 is governed by a formula:

δ=503√(ρ/f*μ _(r).)

The values used here are μ_(r)=1, ρ=16*10⁻⁹ Ωm, and f=27*10⁶ Hz, wherein

-   -   δ=penetration depth (m);    -   ρ=electric conductivity (Ohm m);    -   f=RF frequency (Hz); and    -   μ_(r)=relative magnetic permeability.

An estimation based on this formula indicates that the penetration depthδ at 27 MHz is 12 micrometers, which is the same order of magnitude ofthe size of the silver particles in the metal paste 14 in this example.The relationship between the particle size and the RF frequencynecessary for coupling is established such that particles/objects with atypical size that is much larger than the penetration depth will beheated. Generally, the particle size should be greater than six timesthe penetration depth of the RF field for optimal coupling. However, itis found that RF coupling will work for a particle size equal topenetration depth of the RF field. If the particle size becomes muchsmaller, the efficiency is reduced. Thus, there is an approximate lowerlimit to particle size, e.g. the penetration depth of the RF field.

FIG. 8 illustrates the relationship between penetration depth δ and theRF frequency f for particles of silver, aluminum and nickel. The lowerlimit in particle size that can be heated inductively with a givenfrequency f is determined by the penetration depth, δ=503√(ρ/f*μ_(r)),where ρ and μ_(r) are material properties. For given particle sizes, thefrequency to be used is subject to a minimum, given by the sameequation. For instance:

-   -   15 μm silver particles require a frequency of (more than) 18        MHz;    -   15 μm aluminum particles require a frequency of (more than) 30        MHz;    -   15 μm nickel particles require a frequency of (more than) 0.8        MHz;    -   10 μm silver particles require a frequency of (more than) 40        MHz; and    -   5 μm silver particles require a frequency of (more than) 160        MHz.

Individual excitation of the particles 24 via the RF field heats themetal particles 24 and sinters them together. The efficiency of RFpenetration may be increased because of the additional electricalconnections in the x and y direction, permitting higher currents toflow. A further insight is that the geometry of the metal paste 14deposited on the substrate 12 can be used in modifying temperaturedistribution. FIG. 2 illustrates a side sectional view of a substrate 12for an integrated circuit having an applied pattern of a metalconductive paste 14 under an RF coil 16. When more metal paste 14 ispresent, e.g. when it is thicker, see element A of FIG. 2, thetemperature of the metal paste 14 is reduced. Conversely, element B ofFIG. 2 illustrates a thinner metal paste layer 14 which would heat morequickly in response to RF coupling. Similar effects may be created withdistributions of the metal paste layers 14 in the x- and y-axes.

This principle can be used in specific types of solar cells, forexample. This principle may also be used in other applications, whereina metal layer may be used to shield sensitive parts of the structurefrom the RF field. Alternatively, a multi-layer integrated circuitstructure may be used, as illustrated in FIG. 7. A further insight isthat the dimensions in the z axis, e.g. thickness, of the metal paste 14are determining for the feasibility of the method, and thatpre-sintering of the metal particles of the metal paste 14 increases theefficiency of the method.

A further insight is that the homogeneity of power transfer from the RFenergy 18 into the metal particles of the metal paste 14, e.g. silver,is greatly enhanced by moving the substrate 12 beneath the RF coil 16 ata predetermined rate. The substrate may be moved at a steady or variablerate depending on the desired application of RF energy 18 to each partof the substrate 12 with metal paste 14.

A further insight is that the homogeneity of power transfer from the RFenergy 18 into the metal particles of the metal paste 14, e.g. silver,is greatly enhanced by modifying the RF energy 18 dependent on the exactsubstrate position so that each part of metal paste 14 on substrate 12reaches the same temperature, or a unique temperature desired for thatpart of the substrate 12.

A multi-layered integrated circuit is illustrated in FIGS. 7A-7D. FIG.7A illustrates a side view of a multi-layer integrated circuitarrangement 30 having two metal layers 32, 34 arranged on the substrate12, as shown in FIGS. 7B and 7C. The first metal layer 32 and secondmetal layer 34 may include a metal paste 14, and are shown separated byan insulating layer 36. According to the present invention, the entiremulti-layer integrated circuit arrangement 30 may be subjected to aninductive RF field for simultaneously coupling both the first and secondmetal layers 32, 34. According to the present teachings, the particlesizes for the metal paste 12 and the RF field strength and frequencyshould be selected to ensure sufficient penetration depth.

FIG. 7D illustrates an alternative embodiment of a multi-layerintegrated circuit arrangement 46 wherein the substrate 12 has two metallayers on each side. One side of the substrate 12 includes the first andsecond metal layers 32, 34, separated by the insulating layer 36. Theopposite side of the substrate 12 includes third and fourth metal layers38, 40, separated by an insulating layer 42. The substrate 12 mayinclude a via 44 filled with metal particles. According to the presentinvention, the entire multi-layer integrated circuit arrangement 46 maybe subjected to an inductive RF field for simultaneously coupling all ofthe metal layers 32, 34, 38, 40. The RF field may be provided from twosides of the substrate 12, simultaneously, so as to effectivelyinductively couple all of the metal layers 32, 34, 38, 40. The RF fieldcouples less effectively into the via 44 due to the orthogonalorientation relative to the RF field, so temperature reached in the viais lower than within the 4 metal layers.

In this manner, the device and method disclosed provide an effective andinexpensive way in manufacture integrated circuits having a substratewithin applied metal paste.

While the invention has been illustrated and described in detail in thedrawings and forgoing description, such illustration and description areconsidered to be illustrative or exemplary and not restrictive; theinvention is not limited to the disclosed embodiments. Other variationsto the disclosed embodiments can be understood and effected by thoseskilled in the art in practicing the claimed invention, from a study ofthe drawings, the disclosure, and the appended claims.

In the claims, the word “comprising” does not exclude other elements orsteps, and the indefinite article “a” or “an” does not exclude theplurality. A single processor or other unit may fulfill the functions ofseveral items recited in the claims. The mere fact that certain measuresare recited and neutrally different dependent claims does not indicatethat a combination of these measures cannot be used to advantage. Anyreference signs in the claims should not be construed as limiting thescope.

1. A method for manufacturing an integrated circuit having a thick filmmetal layer comprising the acts of: selectively applying a paste on aheat-conducting substrate using a applicator, the paste comprisingparticles of a metal constituent consisting essentially of metalparticles having sizes within a single predetermined range about asingle predetermined size; and selectively inductively coupling a radiofrequency (RF) energy into the metal particles of the paste from a radiofrequency (RF) generator, the metal particles of the singlepredetermined size are selectively inductively couplable with the RFenergy, for heating the metal particles, wherein the frequency of the RFenergy corresponds to a coupling frequency of the metal particles of thesingle predetermined size so that the inductive heating of the metalparticles is sufficient for melting the metal particles.
 2. The methodclaim 1, wherein the material of the substrate has a higher heatcapacity than the paste comprising the particles.
 3. The method of claim1, wherein the substrate comprises a semi-conductive material comprisingat least one of silicon, gallium-arsenide, germanium, indium-telluriumcompounds and copper-indium-gallium-sulfur compounds.
 4. The method ofclaim 1, wherein the substrate is a heat sink for rapidly cooling theselectively-coupled paste comprising the metal particles.
 5. The methodof claim 1, wherein the metal particles comprise at least one of silver,aluminum, copper and stainless steel.
 6. The method of claim 1, whereinthe coupling frequency is in the range from 2 to 200 MHz, so as to beinductively couplable with the metal particles.
 7. The method of claim1, wherein the metal particles in the paste have a predetermined size inthe range from 5 to 50 μm, so as to be inductively couplable with the RFenergy.
 8. The apparatus of claim 1, wherein the predetermined range ofparticle size of the metal particles is within a range of 1 to 6 times apenetration depth of the RF energy in a material of the metal particles.9. The method of claim 1, comprising moving the substrate having thepaste under the RF generator at a predetermined rate using a substratetable, wherein the RF energy is distributed in a predetermined manner.10. The method of claim 9, wherein the predetermined rate is constantand wherein the predetermined manner is variable and linked to thesubstrate position.
 11. The method of claim 9, wherein the predeterminedrate is variable and wherein the predetermined manner is constant. 12.The method of claim 1, comprising: pre-heating the substrate using asubstrate heater, wherein the conduction properties of the substrate aredependent on temperature.
 13. The method of claim 12, wherein thesubstrate heater directly contacts the substrate.
 14. The method ofclaim 12, wherein the preheating includes heating the substrate above400° C.
 15. The method of claim 12, wherein the substrate material has acritical temperature below which the substrate is not inductively heatedby the radio frequency (RF) for inductively heating the metal particlesand above which the substrate is inductively heated by the radiofrequency (RF) for inductively heating the metal particles to increasethe temperature of the substrate during the inductive heating of themetal particles, the temperature increase of the metal particles beingmore than the temperature increase of the substrate during the inductiveheating, and wherein the preheating heats the substrate above thecritical temperature.
 16. The method of claim 1, wherein the paste isapplied onto the substrate with a predetermined three-dimensionalgeometry, wherein the temperature of the inductively coupled metalparticles depends on the geometry of the paste.
 17. The method of claim1, wherein the coupling frequency is 27 MHz.
 18. The method of claim 1,wherein the size of the particles of the metal constituent in the pasteis selected to maximize the inductive heating of the particles.
 19. Anon-transitive computer readable media encoded with instructions thatwhen communicating with control apparatus for manufacturing anintegrated circuit with a thick film metal layer perform the acts of amethod for manufacturing the integrated circuit having the thick filmmetal layer, the acts comprising: using an applicator to selectivelyapply paste on a heat-conducting substrate, the paste comprisingparticles of a metal constituent consisting essentially of particleshaving sizes within a predetermined range about a predetermined size;and using a radio frequency (RF) generator to selectively inductivelycouple radio frequency (RF) energy into the metal particles of thepaste, the metal particles of the predetermined size are selectivelyinductively couplable with the RF energy, for heating the metalparticles, and wherein the frequency of the RF energy corresponds to acoupling frequency of the metal particles of the paste so that theinductive heating of the metal particles is maximized.